1. Technical Field
The present application relates to a method of machining crankshafts. The present application further relates to a cutting insert for use in the method, which cutting insert has a prismatic basic shape and has a top and a bottom surface which are arranged parallel or relatively parallel to one another and are connected by four side faces which are arranged perpendicularly or relatively perpendicularly thereto. The cutting insert also has two opposite side faces that have projecting and recessed sections, which together with the top and the bottom surface form respective cutting edges, and two other opposite side faces that are arranged in a planar manner and parallel or relatively parallel to one another.
The present application also relates to a method of machining rotated workpieces, possibly crankshafts, in which at least one cutting insert is first being fed in onto the workpiece in the radial direction and then being moved relative to the workpiece in the axial direction.
2. Background Information
Background information is for informational purposes only and does not necessarily admit that subsequently mentioned information and publications are prior art.
Some methods of machining rotationally symmetrical workpiece surfaces, such as crankshafts, involve rotating the workpiece surface during the machining, then first rough-machining by means of cutting tips, and then finish-machining by means of at least one further cutting tip, wherein the workpiece surface to be machined, during the finish machining, is machined by the longitudinal turning process by the further cutting tip moved in the axial direction of the workpiece surface. The tool having the further cutting tip is fed in radially to the workpiece surface during the finish machining, the longitudinal turning being effected by means of at least two lips of the further cutting tip which lie axially next to one another at a distance apart and work essentially simultaneously. The tool used for this purpose has a tool parent body on which at least one cutting tip for the rough machining and at least one further cutting tip for the finish machining are to be arranged. Said further finishing tip has, for the finish machining by longitudinal turning, at least two lips lying next to one another at an axial distance apart, possibly a cutting edge having a tooth-like profile, in which a multiplicity of lips lying next to one another at a distance apart each have a triangular shape. The points formed by these triangular lips lie in one plane.
In other words, one method of machining a crankshaft involves placing a workpiece in a rotating device which holds and rotates the workpiece in the style of a lathe. A cutting tool with a cutting insert or tip is moved radially, i.e. is moved in a direction corresponding to a radius extending perpendicularly to the longitudinal rotational axis of the workpiece, towards and into the workpiece. This first cutting insert engages and cuts a portion of the workpiece as the workpiece rotates as part of a rough machining or cutting process. The first cutting insert is then removed upon completion of the cutting process, and a second cutting tool with another cutting insert, such as a comb-shaped cutting insert, is moved radially toward the workpiece. This second cutting insert also initially contacts the workpiece in a radial direction, however, it is then moved in a substantially axial direction, i.e. in a direction parallel to the longitudinal rotational axis of the workpiece. In this manner the second cutting insert cuts across the face or exterior of the workpiece in a finish machining or cutting process.
Some tools are provided with cutting tips which lie one behind the other in the circumferential direction and with which different machining operations can be carried out on a crankshaft. Rough machining is effected using a first set of, for example, four cutting tips, during which rough machining, in the example cited, cutting inserts which are rectangular in plan view of the rake face are fed in radially in the direction of a crankshaft. Once the crankshaft has been machined by the recessing process to such an extent that a journal circumferential surface having lateral oil collars has approximately developed, a cutting tip having a plurality of triangular cutting edges is fed in onto the workpiece for the finishing and is moved in a longitudinally axial manner for the longitudinal turning.
Some cutting inserts with prismatic basic shapes have two essentially parallel, top and bottom surfaces and at least three side faces which run essentially perpendicularly to the top and bottom surfaces. A cutting edge is formed at the interface between at least one side face and the top or bottom surface. The relevant side face is not planar and has alternately arranged projecting and recessed sections, whereby effective sections of main cutting edges are formed at least in the region of the projecting sections. Extending between these main cutting edges are sloping secondary cutting edges which are angled relative to the main cutting edges partly by 60 degrees and in the corner regions by 45 degrees. Furthermore, the rake faces are formed by recesses assigned to each section of a main cutting edge, an angle between the rake face and the main flank within the range of 62 degrees to 72 degrees being obtained in the central region of a main cutting edge.
These cutting inserts may be fitted on a milling tool in such a way that their main cutting edges overlap slightly and the main cutting edges of one cutting insert mutually cover the recessed regions of a following cutting insert, such that the secondary cutting edges come into engagement with the workpiece with only or substantially only their regions directly or relatively adjoining the main cutting edges. A corresponding tool is provided solely for infeed in the radial direction in order to mill, for example, grooves or the like in a workpiece.
One problem is how to provide a method of machining workpieces in which the cutting insert is to be fed in both radially and axially during the machining of workpieces, wherein the corresponding workpiece surfaces are to be produced efficiently and in good quality, i.e. with low tolerances.
According to one possible method, at least one cutting insert is to be used for producing rotationally symmetrical surfaces. Possibly, this cutting insert is to comprise certain angles, namely a first angle which is formed between the rake faces and the plane of the main flanks, this plane being defined by the regions of the side face adjoining the main cutting edge, and which is to be between 72 degrees and 76 degrees, and furthermore an angle between 75 degrees and 70 degrees between the secondary cutting edges and the main cutting edge and an angle of inclination of the cutting insert at which an effective clearance angle of 8 degrees to 12 degrees is formed. It is emphasized that the selection of such angles firstly makes possible a positive cutting geometry during both the radial and the axial infeed to the workpiece, and in addition reaction forces which occur could be kept relatively small. Such tool is also only suitable for fine machining.
There is therefore the disadvantage with the above-mentioned methods that first, to produce the bearing of a crankshaft, rough machining is carried out using a first cutting insert. To this end, a “triangular indexable insert” having a corner radius of 1.2 millimeters is often used. Then, for the fine machining, one of the cutting inserts described above having a comb-like cutting edge is used, with which the final contour of the bearing root is machined by a longitudinal turning process. In order to be able to produce a crankshaft, a certain number of cutting inserts having different geometries and in different grades should be kept in stock. The number of different types of indexable insert variants is not inconsiderable, which may disadvantageously result in the risk of mix-ups during fitting, with costly malfunctioning of the fitted tools. The multiplicity of cutting inserts desired also results in long setting-up times. Ultimately, the tool costs have a substantial effect on the production costs of the workpieces to be machined.